Volcanic ash clouds constitute a serious hazard to aircraft even after the clouds have moved from the site of a volcanic eruption. Apart from containing ash particles, the clouds include gases such as SO.sub.2, which after a few days oxidises and hydrolises to form sulphuric acid droplets, either as an ash-acid mixture or as a coating over ash particles. Both the ash particles and the sulphuric acid droplets of volcanic ash clouds are capable of causing significant damage to and possible loss of an aircraft which encounters an ash cloud.
A number of aircraft encounters with volcanic ash clouds have been recorded in the past where significant damage has occurred. Between April and July 1982 Galunggung, a volcano in Central Java, Indonesia, underwent a series of eruptions. The dispersing ash clouds, in moving across commercial flight paths, constituted a major hazard because the radar systems of aircraft were, and still are, unable to discriminate between ash clouds and ice/water clouds. Also a volcanic ash cloud normally can not be distinguished visually from a grey ice/water cloud. Two major incidents resulted from aircraft flying directly into the ash clouds. On 24 Jun. 1982, a British Airways Boeing 747, flying from Kuala Lumpur in Malaysia to Perth, entered an ash cloud. All four engines stalled and the aircraft descended 6000 m without power before attempts to restart the engines succeeded. An emergency landing was made at Jakarta, Indonesia. A second incident occurred on 13 Jul. 1982 when a Singapore Airlines Boeing 747 suffered similar inflight problems and again an emergency landing was made at Jakarta. More recently, on 15 Dec. 1989, a KLM Boeing 747 encountered an ash cloud produced by an eruption of the Mount Redoubt Volcano in Alaska, USA. The engines of the aircraft stalled and the jet descended 3600 m without power. Furthermore, the cockpit of the aircraft was reported to have filled with sulphur smelling gases. The descriptions of the encounter were remarkably similar to the Galunggung ash cloud encounters and the effects on the aircraft were also similar.
The above serves to illustrate the requirement for a detection system which could provide a pilot with early warning of the presence of an ash cloud so the cloud can be avoided. A system relying on data collected from satellites could be implemented, however, satellite data for a particular area of the earth's atmosphere is only updated periodically. For example, data from satellites in a polar orbit is only provided four times a day for most regions of the earth. Therefore a detection system which can be installed in an aircraft to monitor conditions ahead of the aircraft is desired. Furthermore, it would be advantageous if the system could also detect other adverse atmospheric conditions, such as a clear air turbulence, which can not be detected visually or by known aircraft detection systems, and should be avoided by aircraft.